Osamu Arimoto

Polariton Luminescence in Monoclinic ZnP2 Crystal

Exciton polariton luminescence and its temperature dependence have been investigated in monoclinic ZnP 2 crystals. The energies of transverse and longitudinal excitons ( E T and E L ) are determined by a fit of the reflection spectrum to a simple polariton model. In the vicinity of the 1 s exciton, two emission bands are observed at energies E T and E L . They are attributed to the luminescence from lower and upper branch polaritons, respectively. In particular, the lower polariton luminescence band exhibits a well-defined doublet structure at 2 K. Both components are separated from each other by about 1 meV with comparable intensities and the same polarization dependence. With increasing temperature, the higher-energy component decreases rapidly as compared to the lower-energy component. The origin of these emission components is discussed.

Intersystem Conversion between Singlet and Triplet Exciton States in ZnP2

Detailed excitation spectra for singlet and triplet luminescence in ZnP 2 are observed at 2 K by using a Ti:sapphire laser. When a singlet exciton is excited, conversion to a triplet exciton occurs over a wide excitation energy range. The spin memory of the exciton is easily lost under the band-to-band excitation condition. When the triplet exciton is excited, the excitation spectrum for the triplet luminescence shows a clear hydrogen-like series up to n =9, which reflects the absorption spectrum of a spin triplet exciton series with s - d doublet structure for n ≥3. The excitation spectrum for the singlet luminescence under the excitation of the triplet exciton is observed for the first time as a new exciton series appearing only for n ≥3. The new exciton series results from the contribution of the d -envelope function of the exciton, and the s -envelope function alone does not give rise to the singlet luminescence.

Temperature Dependence of Exciton Reflection Spectra in Monoclinic Zinc Diphosphide

Reflection spectra near the fundamental edge of monoclinic ZnP 2 crystal have been measured at various temperatures from 2 K to 200 K. As the temperature is raised from 2 to 80 K, minimum value of the reflectivity R min due to the lowest 1s exciton is found to decrease. The R min turns to increase above 80 K. The model calculations of reflection spectra based on the dead layer model are performed for different values of the exciton dissipative damping constants. The analysis of the reflection spectra obtained at various temperatures reveals that this change of the spectra results from monotonical increase of the exciton damping with increasing temperature in the existence of exciton-free “dead layer”. It is suggested that the main contribution to the exciton damping is the scattering by phonons.

Emission from the higher members of exciton (n=2, 3 and 4) in β-ZnP2

Secondary emission spectra from the excited states of exciton series in black monoclinic ZnP 2 single crystal are measured at 6K under the band-to-band excitation condition. The luminescence bands due to the radiative recombination of the 2 s , 3 s and 4 s excitons are clearly observed separately. The changes in intensities of the 1LO Raman line with the energy of 32.2 meV and the luminescence bands with the excitation energy are studied in detail. The 1LO Raman line shows resonance enhancement due to the out-going resonant effect when it comes near the 2 s exciton band. On the contrary, when it is just superimposed on the 3 s luminescence peak the intensities of the 2 s and 3 s luminescence bands and the 1LO Raman line have the minimum values. This weakness is explained with the relaxation of the exciton polariton through the 2LO Raman process.

Resonant Secondary Emission and Its Excitation Energy Dependence in Monoclinic Zinc Diphosphide

Resonant secondary emission of monoclinic zinc diphosphide (β-ZnP 2 ) has been investigated in detail under excitation into the energy range from the interband to the exciton-bands region. In addition to the 1LO line of 32.2 meV already reported, we have found more than 30 resonant Raman lines ranging from 9.4 meV to 58.9 meV by varying the excitation energy. These lines are observed distinctly when their scattered energies fall into the vicinity of the 1s exciton energy. It is confirmed that among phonons of 72 modes the LO phonon of 32.2 meV preferentially governs the relaxation processes of exciton polaritons in β-ZnP 2 . We have also observed a significant intensity decrease of the exciton polariton luminescence when the excitation energy is lowered across the 1s longitudinal exciton energy E L . A brief discussion is made in this connection.

Lifetimes of Singlet and Triplet Excitons in β-ZnP 2

Temporal behaviors of the radiative recombination of excitons have been studied in β-ZnP 2 by using picosecond laser pulses at 2 K. Under interband excitation with E // c , the singlet exciton lumi...

Triplet-to-Singlet Conversion in the Exciton System in β-ZnP 2 - Anti-Stokes Exciton Luminescence -

A new type of experiment was performed to investigate the effective nonradiative process causing the fast decay of the forbidden 1 s triplet exciton luminescence with polarization E // b in β-ZnP 2 . Excitation spectra of anti-Stokes 1 s singlet exciton luminescence with E // c have been measured by varying the excitation energy with E // b around the 1 s triplet exciton absorption peak which is located about 2 meV below the singlet exciton luminescence. A sharp peak of excitation efficiency has been confirmed at the triplet exciton resonance for the first time. Temperature dependence of the excitation peak is in accordance with the thermal behavior of the singlet exciton polariton luminescence both from lower- and upper-branches. This anti-Stokes acoustic phonon scattering of triplet exciton into singlet states is one of the main causes of the short lifetime of triplet exciton in β-ZnP 2 . Spin flip mechanism is briefly discussed.

Time-gated photon counting method for two-photon spectroscopy using synchrotron radiation and laser

A new measurement system has been developed for two-photon spectroscopy in solids with a combination of synchrotron radiation and laser. This system is based on a time-gated photon counting technique to observe luminescence signals induced by two-photon excitation. It is demonstrated that this technique enables us to achieve a high sensitivity and a good signal-to-noise ratio because it is a kind of zero method.

Transient Absorption of Hetero-Nuclear Relaxed Excitons in RbCl: I

Transient absorption due to excitons relaxed in the configuration of [ICl - (V k )+electron] has been identified in RbCl: I under selective excitation into the localized exciton band by using an ArF-excimer laser. The transient absorption spectrum was found to arise from two groups of transitions: electron-transitions appearing in the infrared region and hole-transitions appearing in the ultraviolet region. This result is quite similar to that obtained in the STE absorption in pure alkali halides. The peak location of each absorption band is compared with those of absorption bands due to STEs in RbCl and RbI. Discussion is made on the peak location of hole-transitions in terms of V k -type distortion of ICl - in RbCl matrix.

F-H pair creation from localized exciton in KBr:I

Color center formation has been investigated in KBr:I crystals at 10 K under one photon pulse excitation into the localized exciton band by using ArF-excimer laser light. F and H centers are produced and their absorption spectra agree with those of F and H centers in pure KBr colored with X-rays. From time dependent measurements, it is found that F and H centers are simultaneously created and part of them decay with time constants of ∼20 µs and ∼100 µs by the mutual recombination. The transient behavior of these centers is similar to that of F and H centers in pure KBr. It is concluded that F and H centers are created in pairs from I - -localized excitons and the H centers thus created are not associated with I - ions.